Magnetic disk drives for recording digital data are well known in the art. In the design of such drives, it is usually an objective to achieve the highest recording density possible for a given cost. Recording density is of course dependent on both the number of annular tracks per radial inch of disk surface and the number of bits per inch along a track. The maximum number of tracks which can be recorded per inch is in turn dependent on the precision of the head positioning system. Typically, the head positioning system includes a linear motor connected in a servo loop which also includes position control logic circuitry and a summing amplifier. In use, the address of a destination track is supplied to the logic circuitry which in turn supplies an analog signal to the summing amplifier which develops a positioning signal for application to the linear motor positioner coil. Additionally, one or more offset or compensation signals may be supplied to the summing amplifier to enhance precise positioning. For example, it has been common practice to sense the temperature of the air moving past the disk and supply a signal representative thereof to the summing amplifier in order to offset head positioning to compensate for disk expansion or contraction.
The present invention is useful in disk drives employing both fixed and removable disks. Such drives are well known in the art and find extensive applications where the user requires the permanent storage of some data and the easy interchangeability of other data. In the use of such drives employing both fixed and removable disks, head positioning problems are typically encountered if, after a disk is replaced, operation is attempted without waiting for the removable disk temperature to become substantially equal to the temperature of the fixed disk.